X-ray devices such as CT (computerized tomography) scanners require a power supply capable of delivering to the X-ray tube pulses of DC power that have a short rise time, a high pulse repetition rate (PRR), and high stability (fairly constant peak voltage). A typical requirement is a 120 kV, 300 mA pulse with a 1 ms rise time. The fast rise time is necessary to prevent the damage to living tissue caused by soft X-rays generated as the voltage rises to its peak value. A 10 ms rise time, for example, is unacceptable. Ordinarily, a high voltage tetrode is used for switching the high voltage supply to produce pulses with the required characteristics. Although the high voltage tetrode is capable of producing pulses with a 0.2 ms rise time it suffers from the typical short service life of all vacuum tubes. The high voltage tetrode is also very expensive and requires a large driving circuit.
Another difficulty with conventional high voltage pulsed power supplies has to do with the commercial power source from which the high voltage supply draws its energy. Ordinary power frequencies of 50 or 60 Hz do not permit a high voltage power supply to produce a pulse with a rise time of less than 10 ms. Therefore, it has been proposed that the high voltage power supply include a transformer, in the primary winding of which is a high frequency inverter operating at about 10 kHz. This arrangement, however, has its own set of problems. An X-ray CT scanner requires a pulse whose peak value varies by no more than 1 percent in order to obtain acceptable image quality. Generally, however, the 1 percent maximum variation requirement is not met, for two reasons. First, DC power obtained from a commercial power source and used to drive the high frequency inverter usually includes a ripple component at twice the commercial power supply frequency. Second, the supply voltage gradually drops during the course of operating the X-ray tube.
To solve these problems, it has been suggested that the output of the high voltage pulsed power supply be controlled using negative feedback. Although this control method produces pulses with a fast rise time, it generates over-shoots which damage the X-ray tube.
In other technical fields, such as high speed operation of high capacity motors or brake control, a control method known as the bang-bang control method is used with the automatic control system. In this method, the feedback path is opened or closed by means of a switch. However, the timing of switch operation is so critical that a computer is generally required so as to minimize the evaluation function. Also, the necessary calculations require too much time for this method to be practically applied to control of an X-ray tube.